Aquarium Volume Calculator
Enter your tank dimensions to instantly convert to water volume — litres, US gallons (US gal), UK / imperial gallons (UK gal) and full-tank water weight — for the three most common home shapes: rectangular, cylindrical (column tank) and bow-front (curved front). The default 90 % fill leaves freeboard below the rim and subtracts the volume displaced by substrate, driftwood and décor, giving you a reliable number for planning the cabinet, filter, heater and floor-load.
“Bow protrusion” is how far the curved front glass extends beyond the flat side walls (the sagitta of the curve). For example, a 40 cm back depth with an 8 cm bow puts the front of the curve 48 cm from the back glass.
Enter positive dimensions and a fill percentage between 0 and 100.
Gross volume
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Net water volume
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Water weight (full)
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Freshwater at ~1 L = 1 kg. Glass, cabinet, substrate and rockwork add more — plan the floor load accordingly.
Formula
Rectangular: V = L × W × H Cylinder: V = π × (D/2)² × H Bow-front: V = (W × D_back + A_segment) × H Segment: r = (W² + 4·D_bow²) / (8·D_bow); θ = 2·asin(W/(2r)); A = ½·r²·(θ − sin θ) Net water = V × fillPercent / 100
- · Fresh water at room temperature: ~1 L ≈ 1 kg (actually 0.997 kg/L at 25 °C, < 0.3 % off). We use the “1 L = 1 kg” convention universal in the aquarium hobby.
- · Unit conversions (NIST SP 811): 1 US gallon = 3.785 411 784 L; 1 UK / imperial gallon = 4.546 09 L. Aquariums sold by gallon vary by ~20 % between US and UK conventions — always confirm which one the spec sheet uses.
- · Default fill 90 %: accounts for a 2–3 cm freeboard and displacement by substrate, rocks and driftwood. Bare-bottom display tanks can use ~95 %; heavily aquascaped or reef tanks ~80–85 %.
- · How to measure bow protrusion (sagitta): lay a straight edge across the two side panels and measure to the front-most point of the curved glass. Typical 4-ft bow-fronts have a 5–8 cm bow depth.
- · Full-tank load on the floor ≈ water weight + ~15–20 % for glass (5 mm glass ≈ 12.5 kg/m²) + ~1.5 kg/L for substrate. A 300 L tank fully stocked easily exceeds 400 kg — most residential floors are rated for ~200 kg/m², so spread the load and place it near load-bearing walls.
- · The folk rule of “1 inch of fish per US gallon” is at best a rough starting point; modern stocking guidelines (FishLab, Practical Fishkeeping) weigh species, filtration and oxygenation. This tool sizes water volume only — check a stocking chart separately.
Frequently asked
Why is my actual water volume so much less than the tank’s stated capacity?
Two reasons: (1) the labelled capacity is almost always the gross box volume (L × W × H), with no allowance for glass thickness, internal bracing, the rim and freeboard; (2) once substrate, rocks and aquascape go in, another 10–20 % of usable water is displaced. As a rule of thumb, actual water ≈ stated volume × 85–90 %. The default 90 % fill in this calculator models exactly that — use 95 % or 100 % for a bare-bottom display tank, and 80–85 % for a heavily aquascaped reef or planted setup.
What is the difference between US gallons and UK (imperial) gallons, and which should I use?
A US gallon is 3.785 L; a UK / imperial gallon is 4.546 L — about 20.1 % larger. US gallons are used in the United States, Canada and the Philippines; UK gallons in the United Kingdom, Ireland and several Commonwealth countries. A tank labelled “55 gallon” therefore means ~208 L in the US but ~250 L in the UK — a meaningful difference. This calculator shows both side by side so you can cross-check the manufacturer’s spec. Most of Asia and Continental Europe just quote litres directly.
How do I measure the bow protrusion on a bow-front tank when the spec sheet only gives total dimensions?
The most accurate way is to lay a straight edge or spirit level across the two side panels (the flat back portion) and measure from the straight edge to the front-most point of the curved glass — that distance is the sagitta (bow protrusion). If you only have the manufacturer’s spec sheet, subtract: “total depth − flat back depth = bow”. For example, a 48 cm overall depth with a 40 cm back depth has an 8 cm bow. Typical US 4-ft bow-front tanks have ~5–8 cm of bow; large 6-ft display tanks can have 10–12 cm.
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